The present invention relates to a brake device for a vehicle, and more particularly, to a brake device for a vehicle, which is capable of varying an effective cross-sectional area of a master cylinder.
In general, an electronic hydraulic brake device senses a pedal pressure of a driver through a sensor, and then adjusts a brake pressure of each wheel using a hydraulic modulator.
Such an electronic hydraulic brake device includes the sensor and a pedal travel simulator. The sensor serves to sense a stroke distance of a pedal so as to determine a brake pressure desired by a driver, and the pedal travel simulator serves to make the driver feel a pedal pressure similar to that of a general hydraulic brake device.
Thus, when the driver steps on the brake pedal, the sensor measures a moving distance of the brake pedal, and a master cylinder generates a brake oil pressure according to the operation of the brake pedal.
When a piston of the master cylinder is operated to pressurize brake oil within a cylinder body, the pressurized brake oil is transferred to a wheel cylinder so as to generate a brake force.
The pressure generated from the master cylinder is set according to the inner diameter of the cylinder body and the capacity of a motor.
While the vehicle is operated for common use, the master cylinder requires a relatively low pressure and high response. On the other hand, during an emergency brake situation such as an ABS control situation or brake locking situation, the master cylinder requires a relatively high pressure and low response.
When the motor has a constant capacity, it is desirable that the master cylinder has a large inner diameter in the common-use situation and a small inner diameter in the emergency brake situation.
The related art of the present invention is disclosed in Korean Patent Laid-open Publication No. 1998-0016916 published on Jun. 5, 1998, and entitled “Vapor-lock detecting device of brake master cylinder”
The master cylinder used in the conventional brake device has a single effective cross-sectional area to generate a pressure. Thus, when the pressure generated from the master cylinder is varied, the capacity of the motor must be increased. Thus, the manufacturing cost is inevitably increased, and the size of products is also increased. Therefore, there is a demand for a structure capable of improving the problems.